In a gas turbine engine, air is compressed and combined with a fuel (liquid or gaseous) in a combustion chamber. The air fuel mass mixture is ignited and the energy from the resulting expansion is used to turn a turbine. In such engines, the output power is proportional to fuel flow and, more particularly, is proportional to the sum of the fuel mass flow rate and the air mass flow rate through the combustor.
As a byproduct of the combustion process, gas turbine engines, such as those used to power electrical generators, may emit certain pollutants, such as nitrous oxides (NO.sub.x). NO.sub.x may be limited by controlling the flame temperature of the burners in the combustor. As the flame temperature is increased, the NO.sub.x produced by the engine also increases. Further, if the flame temperature becomes too high, it may damage the combustor or other parts of the engine. In contrast, if the flame temperature becomes too low, the burners may be extinguished, resulting in a "lean blow out". It would, therefore, be advantageous to maintain the flame temperature within a predetermined range to ensure that the NO.sub.x output is limited and that the combustor does not "blow out" or damage the engine.
Combustor flame temperature in a gas turbine engine is proportional to the fuel to air ratio in the combustor. Flame temperature, being proportional to the fuel to air ratio, increases as the fuel to air ratio increases. Therefore, an increase in fuel flow or decrease in airflow through the combustor will normally result in an increase in flame temperature. In contrast, any decrease in fuel flow or increase in airflow will reduce the flame temperature.
It would, therefore, be advantageous to maintain a substantially constant flame temperature by adjusting the fuel to air ratio within the combustor. For example, the airflow to the combustor might be increased as the fuel flow increases to meet increased output power demands. Alternatively the airflow may be decreased as the fuel flow is decreased for reduced output power demand. However, in many gas turbine engines, the airflow is not adjustable over a sufficient range to maintain a substantially constant flame temperature. Therefore, it may be necessary to use other controllable parameters of the engine, such as the burner area, to control the fuel to air ratio.